3rd Generation Partnership (3GPP) Long Term Evolution (LTE) is a project to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a user equipment (UE) 150 is wirelessly connected to a radio base station (RBS) 110a commonly referred to as an eNodeB or eNB (evolved NodeB), as illustrated in FIG. 1. In E-UTRAN, the eNodeBs 110a-c are directly connected to the core network (CN) 190. An LTE system is sometimes also called an Evolved Universal Terrestrial Radio Access (E-UTRA) communication system. In an LTE system, Orthogonal Frequency Division Multiplexing (OFDM) is used in the downlink, i.e. in the transmission from eNodeB to UE, and Discrete Fourier Transform Spread (DFTS) OFDM is used in the uplink, i.e. in the transmission from UE to eNodeB.
The basic LTE downlink physical resource may be seen as a time-frequency grid as illustrated in FIG. 2a, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 2b. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, also called Physical Resource Blocks (PRB), where a resource block corresponds to one slot of 0.5 ms in the time domain and twelve contiguous subcarriers in the frequency domain, as illustrated in FIG. 3a. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station or eNodeB transmits control information including information about to which UEs or terminals data is transmitted, and upon which resource blocks the data is transmitted in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with three OFDM symbols for control signaling is illustrated in FIG. 2c. 
LTE uses Hybrid Automatic Repeat Request (HARQ). After receiving downlink data in a subframe, the UE attempts to decode it and reports to the eNodeB whether the decoding was successful or not. The acknowledgment is sent in form of an ACK when decoding is successful, and in form of a NACK when the decoding is unsuccessful. In case of an unsuccessful decoding attempt, the eNodeB may retransmit the erroneous data.
Uplink control signaling from the UE to the eNodeB comprises, in addition to HARQ acknowledgements for received downlink data:                Scheduling requests, indicating that a UE needs uplink resources for uplink data transmissions; and        UE reports related to the downlink channel conditions, typically referred to as channel status reports, used as assistance for the eNodeB downlink scheduling.        
Such uplink control information is referred to as Layer 1 and Layer 2 (L1/L2) control information. If the UE hasn't already been assigned an uplink resource for data transmission, L1/L2 control information is transmitted in uplink resources specifically assigned for uplink L1/L2 control on a Physical Uplink Control CHannel (PUCCH). As illustrated in FIG. 3a, these resources may be located at the edges of the total available cell bandwidth. Each such resource consists of 12 subcarriers within each of the two slots of an uplink subframe, i.e. a pair of resource blocks or PRBs. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, i.e. one resource consists of 12 subcarriers at the lower part of the spectrum within the first slot of a subframe and an equally sized resource at the upper part of the spectrum during the second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g. in case of a very large overall transmission bandwidth supporting a large number of users, additional resource blocks may be assigned next to the previously assigned resource blocks in the frequency domain.
The reasons for locating the PUCCH resources at the edges of the overall available spectrum are two-fold:                1. Together with the frequency hopping described above, PUCCH resources at the edges of the spectrum maximizes the frequency diversity experienced by the control signaling;        2. Assigning uplink resources for the PUCCH at other positions within the spectrum, i.e. not at the edges, would fragment the uplink spectrum making it impossible to assign very wide transmission bandwidths to a single UE and still retain the single-carrier property of the uplink transmission        
However, the bandwidth of one resource block during one subframe is too large for the control signaling needs of a single UE. Therefore, to efficiently exploit the resources set aside for control signaling, multiple terminals may share the same resource block pairs. This is done by assigning the different UEs different orthogonal phase rotations of a cell-specific length-12 frequency domain sequence and/or different orthogonal time-domain cover codes covering the symbols within a slot or subframe.
There are different PUCCH formats defined in the 3GPP LTE standard to handle the different types of uplink control signaling. In LTE Rel-8, a PUCCH format 1 resource is defined and used for either a HARQ acknowledgement or a scheduling request. PUCCH format 1 is capable of at most two bits of information per subframe. As a channel status report consists of multiple bits per subframe, PUCCH format 1 may obviously not be used for signaling channel status reports. Transmission of channel status reports on the PUCCH is instead handled by PUCCH format 2, which is capable of multiple information bits per subframe. There are actually three variants of this PUCCH format: PUCCH format 2, PUCCH format 2a, and PUCCH format 2b. The will hereinafter all be referred to as PUCCH format 2 for the sake of simplicity.
However, with the introduction of carrier aggregation (CA) in LTE Rel-10, a new PUCCH format is needed. In LTE Rel-10 the total available spectrum may be wider than the maximum 20 MHz LTE carrier corresponding to the total available spectrum in Rel-8, and may appear as a number of LTE carriers to an LTE Rel-8 UE. Each such carrier may be referred to as a Component Carrier (CC) or a cell. To assure an efficient use of a wide carrier also for legacy UEs, CA is used implying that an LTE Rel-10 UE may receive multiple CCs, where the CCs have or at least are enabled to have the same structure as a Rel-8 carrier. CA is schematically illustrated in FIG. 4, where five CCs of 20 MHz provides a total aggregated bandwidth of 100 MHz. However, another use case for CA is when an operator makes use of smaller parts of bandwidths in different frequency bands, or within a same frequency band, to get one larger aggregated bandwidth. With CA, a PUCCH format that enables feedback of multiple HARQ bits corresponding to multiple CCs is needed. Such a PUCCH format is in the following referred to as PUCCH format 3. However, PUCCH format 3 may also be referred to as CA PUCCH format or DFTS-OFDM PUCCH format.
Sounding Reference Signals (SRS) transmitted by the UE may be used by the base station to estimate the quality of the uplink channel for large bandwidths outside the span assigned to a specific UE. SRS are configured periodically in a subframe, and are transmitted in the last DFTS-OFDM symbol of the subframe. This implies the need of both a normal PUCCH format 3 to use when no SRS are transmitted in the subframe, and a shortened PUCCH format 3 which is muted in the last DFTS-OFDM symbol of the subframe to avoid collisions with SRS transmissions when they are transmitted in the subframe. The amount of UEs that may share the PUCCH format 3 resource may therefore vary depending on if the shortened or the normal PUCCH format 3 is used.